Custom qlog filters

Currently quiche's `QlogStreamer` only supports 3 fixed `QlogLevels` (`Core`, `Base`, `Extra`).

The QUIC layer alone already logs a lot of events, and upper layers like H3 or MoQ add their own events on top. Enabling all of them causes significant I/O overhead and slows down execution.

For debugging/experimentation, often only a specific subset of qlog events is of interest — potentially spanning multiple layers, not just one importance tier. I'd like `QlogStreamer` to support custom filter functions, so users can select exactly which events get logged instead of being limited to the three coarse levels.

Start by locating QlogStreamer and its existing QlogLevels implementation, then trace how qlog events are selected and find the relevant tests or examples. Done means users can choose an arbitrary subset of events across layers without being limited to Core, Base, or Extra, with coverage showing the filtering behavior.

Written by the indexing model from the issue text.